Inline Pond Treatment Performance and Longitudinal Instream Monitoring in a Stormwater-Affected Urban Waterway

Addington Brook in Christchurch, receives stormwater runoff from a 246 ha catchment of mixed industrial/commercial/residential land use. Instream water quality monitoring has shown elevated suspended solids, metal and nutrient concentrations along the brook, which is thought to be a major contributor of these pollutants to the Ōtākaro/Avon River and downstream estuary. Water quality and flow data from four different sites in Addington Brook during storm events were analysed to a) evaluate the performance of an inline, 18-year-old stormwater treatment pond in the upper catchment, b) evaluate spatial and temporal relationships between rainfall, instream flow and pollutant concentrations along the stream that could help prioritise mitigation strategies in different areas of the catchment, and c) evaluate the ability of the Modelled Estimates of Discharges for Urban Stormwater Assessment (MEDUSA) pollutant load model to predict pollutant source contributions. The pond performance analysis revealed significant (p 0.0001-0.04) removal of dissolved metals, solids and nutrients, demonstrating the ongoing effectiveness of pond systems for removing dissolved pollutants. However, discharge of untreated stormwater downstream of the ponds resulted in higher pollutant concentrations in the lower catchment. Pollutant rates of up to 5.2 g/min zinc (Zn), 0.2 g/min copper (Cu), 53 mg/min lead (Pb), 25 kg/min total suspended solids (TSS), 121 g/min total nitrogen and 28 g/min total phosphorus were measured where the brook converges with the Ōtākaro/Avon River. Dissolved Zn concentrations (by an order of magnitude), and to a lesser extent dissolved Cu, consistently exceeded the relevant guidelines for healthy urban waterways at all sampling sites, substantiating an earlier study that used MEDUSA to identify sources of high solids and metal loads in this catchment. The similarity of measured pollutant loads to the MEDUSA-predicted loads indicates that MEDUSA is effective at predicting the amount and origin of dissolved Zn and dissolved Cu in the catchment.